The present invention relates to an ion implantation apparatus and a tuning method for an ion source system thereof.
In recent ion implantation apparatuses, with the shrinking of semiconductor devices, the energy level for implanting ions is being lowered to reduce the depth of ion implantation. In a lower energy range, however, the extraction voltage from an ion source is lower. This has been causing ion extraction efficiency to deteriorate and ion beams to repel one another due to the electric charges thereof with consequent divergence of ion beams, which is known as space charge effect. Hence, there has been a problem in that the degraded transporting efficiency prevents sufficient implantation ion beam current from being obtained.
The above problem will be described with reference to the accompanying drawings.
FIG. 1 shows the construction of a beam line from an ion source of an ion implantation apparatus to a mass analysis slit. In FIG. 1, an ion beam 5 extracted from an ion source 1 through the intermediary of an extraction electrode 2 is subjected to the mass analysis by a mass analysis magnet 3 and a mass analysis slit 4 located at downstream side of the mass analysis magnet 3 thereby to select only required ion species.
FIG. 2 shows the construction of the extraction unit of the ion source used with the ion implantation apparatus. The descriptions will be given of an ion source for taking out ions carrying positive electric charges. In FIG. 2, a positive voltage is being applied to an entire ion source 6. The distal end portion of the ion source 6 has an arc chamber 7 for generating plasma 8. The arc chamber 7 includes an opening 8a for extracting ions from the plasma 8.
Although not shown in FIG. 2, a magnet or a source magnet that acts to generate a magnetic field for efficiently generating plasma in the arc chamber 7 is installed outside the arc chamber 7.
The extraction electrode for extracting ions is generally constructed by a plurality of electrodes each having a slit. Of the plurality of electrodes, the last-or post-electrode as observed from the ion source 6 is usually referred to as a ground electrode 9. As a whole, ions 13 are extracted from the plasma 8 in the arc chamber 7 by an extraction electric field directed from the positive potential of the ion source 6 to the ground electrode 9. At the same time, the ions are accelerated to a desired level of extracting energy.
A suppression electrode 10 is provided on the upstream side of the ground electrode 9. The suppression electrode 10 is subjected to a negative potential with respect to the ground electrode 9 so as to form a negative voltage barrier. The negative voltage barrier prevents the extraction electric field from inversely accelerating ions, that is, accelerating electrons from the ground electrode 9 toward the ion source 6, while the extraction electric field should accelerate ions from the ion source 6 toward the ground electrode 9. Thus, the suppression electrode 10 serves to form the negative voltage barrier for minimizing the chance of electrons from going out into the extraction electric field.
The extraction electrodes, namely, the suppression electrode 10 and the ground electrode 9, are usually secured by a supporting member to a vacuum chamber or the like that accommodates the extraction electrodes. In FIG. 2, however, the extraction electrodes and the vacuum chamber are separately provided. More specifically, the extraction electrodes are supported by a supporting member 12 connected to a driving mechanism 11. This allows the extraction electrodes to be moved in the longitudinal direction (in the direction of the gap axis), i.e., in the upstream or downstream direction of an ion beam stream, and also in the lateral direction (in the direction of the side axis), i.e., the direction orthogonal to the ion beam stream, thus permitting its positional relationship with the arc chamber 7 to be adjusted. In some cases, adjusting devices for tilt axis adjustment and vertical axis adjustment may be added. The tilt axis adjustment is performed to adjust the tilt angles of the extraction electrodes with respect to a central axis in the same direction as that of the gap axis. The vertical axis adjustment is performed to adjust the vertical movement in the direction orthogonal to the ion beam stream.
FIG. 3 schematically illustrates the electrical potential of the extracting system. In the zone from the ion source to the suppression electrode, an ion 14 is accelerated from an ion source potential 15, which is a high potential, toward a suppression electrode potential 16, which is a low potential. After the suppression electrode potential 16, the ion 14 is decelerated to a ground electrode potential 17. Hence, the ion energy (keV) upon completion of the extracting operation will take the value obtained by multiplying the voltage difference {extraction voltage (kV)} between the positive potential applied to the ion source and the potential of the ground electrode (generally a ground potential) by the valence of the ion.
The ion having the desired energy obtained by passing through the ground electrode is transported to a mass analysis magnet, which is the next destination.
The potential gradient from the ion source to the suppression electrode is known as an extraction electric field, and directly influences the extraction of ions from an arc chamber. As shown in FIG. 2, the zone wherein the extraction electric field acts, i.e., the zone extending from the ion source 6 or the arc chamber 7 to the suppression electrode 10, is referred to as an xe2x80x9cextraction gapxe2x80x9d for convenience. When the size of the extraction gap is fixed, the gradient of potential becomes more gentle as the extraction voltage decreases. In other words, as the extraction voltage decreases, the extraction electric field becomes lower. On the other hand, when the extraction voltage is fixed, the gradient of potential grows steeper as the size of the extraction gap is reduced, making it possible to increase the extraction electric field.
Thus, the size of the extraction gap is an extremely important factor directly related to the efficiency of extracting ions from an ion source. For this reason, as shown in FIG. 2, the ion extracting system of a typical ion implantation apparatus has the driving mechanism 11 for moving the extraction electrode thereby to permit the adjustment of the size of the extraction gap. A gap axis is used primarily for adjusting the aforesaid extraction electric field. A side axis and a tilt axis are used to make fine adjustment for aligning the direction of an ion beam to be extracted with a design beam axis.
The energy of ions depends upon the voltage difference (extraction voltage) between the positive potential applied to the ion source and the potential of the ground electrode (generally the ground potential). Therefore, to take out low-energy ions, the voltage of the ion source has to be reduced. For example, to extract 80 (keV) ions by monovalent ions, a voltage of 80 (kV) is applied to the ion source. To extract 0.5 (keV) ions by monovalent ions, the voltage of only 0.5 (kV) can be applied to the ion source.
If the voltage applied to the ion source is decreased with the extraction gap size remaining unchanged, the extraction electric field applied to the extraction gap weakens. As a result, the ion extraction efficiency deteriorates with a consequent reduction in ion current that can be taken out. To avoid this, when low-energy ions are extracted, adjustment is performed by the driving mechanism to reduce the size of the extraction gap so as to bring the suppression electrode and the ground electrode closer to the ion source. In other words, the deterioration in the extraction efficiency is compensated for by controlling the weakening of the extraction electric field.
Ions are characterized by their tendency to repel each other because of their own positive electric charges and consequently diverge. The phenomenon in which the ions diverge due to their own electric charges is known as the space-charge effect. For the same ions and the same electric current, the space-charge effect is intensified at lower energy. Because of the diverging phenomenon, as the energy level becomes lower, the loss of ions increases when the ions advance for the same distance while diverging. This means that the ion beam transporting efficiency degrades. A shorter distance between the extraction electrodes and the mass analysis magnet is better to effectively transport the ion beam extracted from the ion source to the mass analysis magnet.
As described above, however, the suppression electrode and the ground electrode are moved toward the ion source so as to make up for the deterioration in the efficiency for extracting from the ion source caused by lower energy. This poses a problem in that the distance for transporting lower energy ions is inevitably increased. The distance over which the low energy ions are transported is defined as the distance from the ground electrode to the mass analysis magnet. The distance is denoted by A in FIG. 1.
As previously described, narrowing the extraction gap, i.e., the distance between the ion source and the extraction electrodes, results in the undesirable side effect of deteriorated efficiency of transporting the low-energy ions from the ground electrode to the mass analysis magnet. This has been preventing efficient acquisition of low-energy ions.
In the ion implantation apparatus, the mass analysis slit is disposed at the point where an ion beam that has left the mass analysis magnet converges, thereby removing ions that have different masses or energy levels. It has been known, however, an undesirable side effect, in which the ion beam converging point is dislocated, results if the suppression electrode and the ground electrode are moved toward the ion source to narrow the extraction gap.
The mass analysis magnet has an intrinsic focal length. Changing the position of the ground electrode, which is the starting point of the transport of an ion beam, will accordingly change the converging point of the ion beam that forms an image again after leaving the mass analysis magnet. This is illustrated in FIG. 4.
FIG. 4 shows the ion having left the ground electrode 19 is subjected to a change in its trajectory by the mass analysis magnet, and carried to a mass analysis slit 20. A solid line 21 denotes the diverging ion beam when the ground electrode 19 is positioned at point B, and the converging point of the ion beam is denoted by point BF. In this case, the position of the converging point substantially coincides with the position of the analysis slit 20. This allows the desired ions to efficiently pass through the analysis slit 20.
The diverging ion beam observed when the ground electrode 19 is positioned at point C is denoted by a dashed line 22, and the converging point of the ion beam is denoted by point CF. In this case, the position of the analysis slit 20 and the converging point CF do not coincide, so that some of the desired ions collide against portions other than the slit of the analysis slit 20 and therefore are lost.
Thus, the dislocation of the converging point prevents some of the ion beam from passing through the analysis slit 20, causing the ion beam current to reduce. Furthermore, in the operation for optimizing only the extraction efficiency by adjusting the extraction gap, it will be difficult to achieve optimum tuning of the ion implantation apparatus because an increase or decrease in the beam current is added due to the loss of the ion beam in the analysis slit 20.
Furthermore, if the converging point is dislocated, some of the ions having different masses or energy levels that should be separated will remain, causing deteriorated mass resolution.
FIG. 5B shows the state wherein some ions that should be separated pass through the analysis slit because of a dislocated converging point. As illustrated in FIG. 5A, when the converging point substantially coincides with the position of an analysis slit 24, the desired ions are able to efficiently pass through the analysis slit 24, as indicated by solid lines 25. Unwanted ions having different curvature radii cannot pass through the slit 24, so that they are separated from the rest, as indicated by dashed lines 26.
In FIG. 5B, if the converging point is dislocated from the position of the analysis slit 24 due to a change of the position of the ground electrode 23, then some of the unwanted ions that should be separated, as indicated by a dashed line 26, will pass through the analysis slit 24 whereas only the desired ions should pass therethrough.
As described above, the method whereby the ion extraction unit is tuned by moving the extraction electrodes presents the following problem.
A. In the case of low-energy ions, the ion beam transporting distance to the mass analysis magnet increases, with resultant lower ion beam transporting efficiency. This leads to a reduction in the beam current that can be used.
B. The tuning is difficult because the beam current increases or decreases due to a change in the position of the converging point of an ion beam after leaving a mass analysis magnet.
C. The mass resolution degrades also because the position of the converging point of an ion beam changes after leaving the mass analysis magnet.
Accordingly, it is an object of the present invention to make it easier to obtain ion beam current in a low energy range in an ion implantation apparatus.
To this end, an ion implantation apparatus according to the present invention comprises an ion source for generating ions, and an extraction electrode for extracting ions from the ion source by the action of an extraction electric field. The trajectory of an ion beam extracted by the extraction electrode is deflected or bent by the mass analysis magnet. The ions that have passed through the mass analysis magnet are implanted into a target.
According to an aspect of the present invention, the ion implantation apparatus further includes a first driving mechanism for moving the ion source. With this arrangement, the relative positional relationship between the ion source and the extraction electrode can be changed.
Preferably, the first driving mechanism is capable of displacing the ion source in the direction of a gap axis that is the same direction as an ion beam direction, in a vertical or side axis direction right-angled to the ion beam direction, and in a tilt axis direction at an angle with respect to a central axis in the same direction as the ion beam direction.
The ion implantation apparatus further includes a source magnet for generating a magnetic field in the ion source. Preferably, the first driving mechanism moves, in synchronization with the movement of the ion source, the source magnet in the same direction in which the ion source is moved.